Page 1 of 2 12 LastLast
Results 1 to 30 of 45

Thread: Is there a minimum mass to the universe, according to the anthropic principle?

  1. #1
    Join Date
    Nov 2004
    Posts
    5,530

    Is there a minimum mass to the universe, according to the anthropic principle?

    I'm pretty sure there's a maximum mass, after which the gravity would overwhelm expansion, but is there a minimum mass? Could a universe of just one star exist?
    "Occam" is the name of the alien race that will enslave us all eventually. And they've got razors for hands. I don't know if that's true but it seems like the simplest answer."

    Stephen Colbert.

  2. #2
    Join Date
    Aug 2008
    Location
    Wellington, New Zealand
    Posts
    4,278
    I would say there is a minimum mass needed but not 1 star. A universe that forms 1 star might not have a second generation of stars with more metals (elements that are not H) or a third generation with even more metals. The Sun is a second or even third generation star. We are made of star dust from the earlier generations of stars. So the anthropic principle suggests lots of stars and so maybe a small galaxy would be closer to the minimum mass.

  3. 2019-Oct-07, 09:07 AM

  4. #3
    Join Date
    Apr 2005
    Posts
    2,489
    From the anthropic principle, it would be correct to say the observed universe is more or less the minimum mass necessary.
    The universe as it is, is the minimum requirement for intelligent life to evolve ?

  5. #4
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Quote Originally Posted by Reality Check View Post
    I would say there is a minimum mass needed but not 1 star. A universe that forms 1 star might not have a second generation of stars with more metals (elements that are not H) or a third generation with even more metals. The Sun is a second or even third generation star. We are made of star dust from the earlier generations of stars. So the anthropic principle suggests lots of stars and so maybe a small galaxy would be closer to the minimum mass.
    Just to be picky, could not one star go supernova? In the absence of other stars and dark matter, it would reform due to gravity and cycle as many times as necessary? In which case the minimum mass is somewhere near one large star?
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  6. #5
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    37,105
    Quote Originally Posted by kzb View Post
    From the anthropic principle, it would be correct to say the observed universe is more or less the minimum mass necessary.
    The universe as it is, is the minimum requirement for intelligent life to evolve ?
    How do we know our Universe is minimal? What if it's the maximum mass?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  7. #6
    Join Date
    Sep 2003
    Posts
    12,872
    Quote Originally Posted by parallaxicality View Post
    I'm pretty sure there's a maximum mass, after which the gravity would overwhelm expansion, but is there a minimum mass? Could a universe of just one star exist?
    Assuming you mean no other initial condition is changed, then the amount of mass/energy, IIRC, is very, very tightly constrained. A tiny bit more, and collapse would quickly occur. A tiny bit less and stars fail to form. I think this is right.
    We know time flies, we just can't see its wings.

  8. #7
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Quote Originally Posted by George View Post
    Assuming you mean no other initial condition is changed, then the amount of mass/energy, IIRC, is very, very tightly constrained. A tiny bit more, and collapse would quickly occur. A tiny bit less and stars fail to form. I think this is right.
    Given that, you could draw a bubble around a volume that is destined to form a large star and treat that as a lone star, even if that volume is not the same as is needed in a distributed universe?
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  9. #8
    Join Date
    Sep 2003
    Posts
    12,872
    Quote Originally Posted by profloater View Post
    Given that, you could draw a bubble around a volume that is destined to form a large star and treat that as a lone star, even if that volume is not the same as is needed in a distributed universe?
    I think I understand your question, but the Jean's Instability (addresses star formation) is more about density than volume so if the initial mass/energy was slightly less, then I think the idea is that the density would not be adequate to allow collapse to be triggered (supersonic flows or SNs) in order to be in the star making business.
    We know time flies, we just can't see its wings.

  10. #9
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,789
    Quote Originally Posted by profloater View Post
    Just to be picky, could not one star go supernova? In the absence of other stars and dark matter, it would reform due to gravity and cycle as many times as necessary? In which case the minimum mass is somewhere near one large star?
    My bold. I don't think so. If I am not mistaken the ejecta from a supernova is going vastly faster than escape velocity.

  11. #10
    Join Date
    Jan 2010
    Location
    Wisconsin USA
    Posts
    2,890
    All speculation. It is likely the universe is finely tuned so that the minimum and maximum are exactly the same.
    The moment an instant lasted forever, we were destined for the leading edge of eternity.

  12. #11
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Quote Originally Posted by Hornblower View Post
    My bold. I don't think so. If I am not mistaken the ejecta from a supernova is going vastly faster than escape velocity.
    There is no escape velocity in radial expansion but i can see that you could get interactions leading to spin. Then the returning particles would have complex motions and make smaller stars, planets and dust. The universe we observe has distributed stars planets and dust many times over. I just speculate that ignoring the creation event, one big star might be enough. It’s a small bang hypothesis.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  13. #12
    Join Date
    Apr 2005
    Posts
    2,489
    Quote Originally Posted by profloater View Post
    There is no escape velocity in radial expansion but i can see that you could get interactions leading to spin. Then the returning particles would have complex motions and make smaller stars, planets and dust. The universe we observe has distributed stars planets and dust many times over. I just speculate that ignoring the creation event, one big star might be enough. It’s a small bang hypothesis.
    If one big star really was enough, then that is what our universe would consist of. Since it is much bigger than that, there is some reason or reasons why one star is not enough. We don't even need to know what those reasons are.

  14. #13
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Quote Originally Posted by kzb View Post
    If one big star really was enough, then that is what our universe would consist of. Since it is much bigger than that, there is some reason or reasons why one star is not enough. We don't even need to know what those reasons are.
    Is that a restatement of the anthropic principle? Otherwise it's just an untestable belief. Although I am quite happy to believe in the Goldilocks metaphor, can we test that?
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  15. #14
    Join Date
    Jul 2018
    Posts
    115
    Quote Originally Posted by parallaxicality View Post
    I'm pretty sure there's a maximum mass, after which the gravity would overwhelm expansion, but is there a minimum mass? Could a universe of just one star exist?
    Well that depends on what your goal is. If it's simply to have a universe, ANY universe, then I suppose a single particle would technically be enough to qualify for that. So I don't think there is a minimum mass to just have a universe exist.

    However, if you want a universe with stars in it then I would assume you'd need a LOT more then just 1 solar mass worth of stuff since it's highly unlikely that after the big bang 100% of the material in the universe will collapse into the same object. So you'd probably need a lot more then 1 solar mass to get a star to form.

    BUT.....that would also most likely lead to many, many stars forming and not just 1. So if your goal is to literally have a universe with exactly ONE star and that's it, then I don't really see how that could be accomplished other then through some incredible fine tuning.

    Or if you are asking what is the minimum mass needed for life similar to Earth to evolve, my personal guess would be enough mass to create a galaxy. You need a galaxy for second and third generation stars to make the heavy elements like carbon and oxygen. But you probably don't need much more then one galaxy for that. So at this point you'd then need to figure out if a universe with the mass of only the Milky Way would slow the expansion enough to even form stars at all, and that's a question I don't know the answer to.


    Quote Originally Posted by profloater View Post
    There is no escape velocity in radial expansion
    There isn't? I may be misunderstanding what you meant to say, but I don't see why a single star wouldn't have an escape velocity. The Earth certainly has an escape velocity, if something is launched at, say, 50km/s away from the Earth it will never fall back, even if you let it travel to infinity. Why wouldn't this also be true of a star going supernova?

  16. #15
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Quote Originally Posted by Dave241 View Post
    Well that depends on what your goal is. If it's simply to have a universe, ANY universe, then I suppose a single particle would technically be enough to qualify for that. So I don't think there is a minimum mass to just have a universe exist.

    However, if you want a universe with stars in it then I would assume you'd need a LOT more then just 1 solar mass worth of stuff since it's highly unlikely that after the big bang 100% of the material in the universe will collapse into the same object. So you'd probably need a lot more then 1 solar mass to get a star to form.

    BUT.....that would also most likely lead to many, many stars forming and not just 1. So if your goal is to literally have a universe with exactly ONE star and that's it, then I don't really see how that could be accomplished other then through some incredible fine tuning.

    Or if you are asking what is the minimum mass needed for life similar to Earth to evolve, my personal guess would be enough mass to create a galaxy. You need a galaxy for second and third generation stars to make the heavy elements like carbon and oxygen. But you probably don't need much more then one galaxy for that. So at this point you'd then need to figure out if a universe with the mass of only the Milky Way would slow the expansion enough to even form stars at all, and that's a question I don't know the answer to.




    There isn't? I may be misunderstanding what you meant to say, but I don't see why a single star wouldn't have an escape velocity. The Earth certainly has an escape velocity, if something is launched at, say, 50km/s away from the Earth it will never fall back, even if you let it travel to infinity. Why wouldn't this also be true of a star going supernova?
    Escape velocity is to go into orbit. In a single star example radially ejected mass will decelerate due to the star gravity and in the absence of other masses, will halt and fall back. Photons trajectories then depend on your one star universe curvature! But to make more elements for future stars it is the mass ejection we need.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  17. #16
    Join Date
    Nov 2004
    Posts
    5,530
    Quote Originally Posted by kzb View Post
    If one big star really was enough, then that is what our universe would consist of. Since it is much bigger than that, there is some reason or reasons why one star is not enough. We don't even need to know what those reasons are.
    All the anthropic principle says is that we can exist in a universe this massive. Not that we have to. The range of possible alternatives may be very large, or very small.
    "Occam" is the name of the alien race that will enslave us all eventually. And they've got razors for hands. I don't know if that's true but it seems like the simplest answer."

    Stephen Colbert.

  18. #17
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,789
    Quote Originally Posted by profloater View Post
    Escape velocity is to go into orbit. In a single star example radially ejected mass will decelerate due to the star gravity and in the absence of other masses, will halt and fall back. Photons trajectories then depend on your one star universe curvature! But to make more elements for future stars it is the mass ejection we need.
    I'm sorry, but you appear to be misunderstanding what escape velocity is all about. In my entry level physics courses in high school and college, escape velocity at any given distance from the center of the central body was the velocity at which the kinetic energy of the ejected object equals its gravitational potential energy. In that case the object moves away on either a parabolic trajectory or a radial straight line and never returns, with the velocity asymptotically approaching zero as it recedes. For higher initial velocity the object asymptotically approaches a non-zero terminal velocity as it recedes.

    Here is a source I found somewhat helpful on the topic of supernova blasts.

    https://ntrs.nasa.gov/archive/nasa/c...9720004110.pdf

    It appears that with a core-collapse supernova in which the remnant includes a neutron star at the center, some of the material near the center does fall back in and accretes on the neutron star, but the total energy available for accelerating ejecta far exceeds the gravitational binding energy. Thus the stuff that did not fall back in will disperse far and wide in a thought-exercise vacuum, never to return. Of course in the real world there is interstellar medium matter that can stop some of it. In our thought exercise with one star and nothing else, I would not expect that dispersed material to reform into new stars.

  19. #18
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    37,105
    Quote Originally Posted by profloater View Post
    Escape velocity is to go into orbit.
    Escape velocity is to break orbit. Isn't it?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  20. #19
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,789
    Quote Originally Posted by Noclevername View Post
    Escape velocity is to break orbit. Isn't it?
    That is one way of looking at it. A satellite in a stable closed orbit is moving at less than the escape velocity for its current altitude.

  21. #20
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Yes I was too brief, but in this scenario we are talking radial ejection, not launching into orbit. Any radial ejection is doomed to fall back unless there are other bodies to grab the matter. Most of a supernova explosion must be approximately radial so most of it will fall back eventually. And the fall back will bring the energy back too, excepting the photon energy. If there is tangential ejection it will go into orbit or escape thanks to its rotation relative to the remnants of the original star.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  22. #21
    Join Date
    Oct 2005
    Posts
    26,778
    Quote Originally Posted by profloater View Post
    Any radial ejection is doomed to fall back unless there are other bodies to grab the matter.
    No, any radial ejection faster than the local escape speed implies the object has a net positive total (kinetic plus potential) energy, in a system where the potential energy is zero at infinity and negative everywhere else (the standard convention). Since energy is conserved, it will always have a total positive energy, which means its kinetic energy will always stay positive, which means its radial velocity will never be zero, which means it will always stay positive, which means it can never fall back down. There is no need for any tangential velocity to get escape, you can think entirely in terms of having a positive total energy. Indeed, any tangential velocity will eventually turn completely radial if the total energy is positive.

  23. #22
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,789
    Thanks, Ken G, for doing the mathematical legwork while I was away from home. I would have said it in much the same way.

    An interesting detail here is that the magnitude of the escape velocity is independent of the launch direction. Of course the equation of motion will be different at different launch angles, as will the elapsed time required to reach a given separation, but the scalar speed of the ballistic body at any separation depends only on the separation and the initial speed.

  24. #23
    Join Date
    Oct 2005
    Posts
    26,778
    Right, they don't launch in the direction of Earth rotation in order to lower the escape speed, they do it in order to increase their own speed. All that's needed is positive total energy.

  25. #24
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    OK I was wrong, but in the single star case there must be a gravitational deceleration. I guess it drops off with the square of distance while the kinetic energy stays high enough to keep going. My mistake.

    So only matter ejected below escape velocity for the big star comes back while a shell of escapees is now part of the new universe. At some size the escape velocity will become super luminal though, if I got that right, so there is a size for a cyclic star supernova. With the mass now expanding in a big ball, the situation is still complex and I still make no claims as-to how to create this singularity.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  26. #25
    Join Date
    Sep 2003
    Posts
    12,872
    Quote Originally Posted by profloater View Post
    OK I was wrong, but in the single star case there must be a gravitational deceleration. I guess it drops off with the square of distance while the kinetic energy stays high enough to keep going. My mistake.
    Yes, outside the star it drops off by the square of the radial distance. Escape velocity comes from the velocity of KE. Using the conservation laws (see Wiki on escape vel.) [Gravity variation is different within a mass since some mass is overhead countering the lower mass.]

    (K + U)initial = (K + U)final. K is KE and U is gravitational energy. The initial conditions is the blast from the surface radially outward, and the final case is where it goes, for math purposes, to infinity (Lim --> inf.), where KE = 0 and U is also 0. So,
    m*Ve2/2 = G*Mearth/Rearth
    Ve = (2GM/r)1/2

    Given a few prior posts, I was curious what the Ve change would be if we shot out to L1 to the Moon. That would be the new U. I used PE at that L1 point and, probably erroneously, that the Ve would only be about 14% without the Moon. [That might be right because it doesn't boost the speed enough to get it near infinity, but, if right, it's kind of surprising, to me at least.]
    We know time flies, we just can't see its wings.

  27. #26
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,789
    Quote Originally Posted by profloater View Post
    OK I was wrong, but in the single star case there must be a gravitational deceleration. I guess it drops off with the square of distance while the kinetic energy stays high enough to keep going. My mistake.

    So only matter ejected below escape velocity for the big star comes back while a shell of escapees is now part of the new universe. At some size the escape velocity will become super luminal though, if I got that right, so there is a size for a cyclic star supernova. With the mass now expanding in a big ball, the situation is still complex and I still make no claims as-to how to create this singularity.
    My bold. I don't see how that follows from what is written before it. From everything I have read about supernovae it is a once-and-done event. Of course in the real world the ejecta slams into surrounding clouds of gas and dust and triggers star formation, some of which could be future supernovae. That is not what we have in this thought exercise with a universe consisting of a single evolving star and nothing else.

  28. #27
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    Well the Op is about a minimum universe. We have a many stars universe with first series stars going supernova making the higher elements. As you say they disperse to other stars. That makes sense for escape velocity matter. But I am just imagining a fall back of lower velocity into a reformed star. I do not want to turn this into an ATM suggestion, it’s just a question prompted by the OP question. Could a large first series star cycle itself to make the elements neededfor planets?
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  29. #28
    Join Date
    Oct 2005
    Posts
    26,778
    It seems to me the issue of the minimum mass universe to have intelligent life must rest on how likely is intelligent life, given a universe that can already do the mundane things like make galaxies. So it's not so much about whether it can make heavy elements, it's whether or not it can make a vast number of stars. Maybe a single galaxy is enough, maybe it isn't-- that's really what we don't know.

  30. #29
    Join Date
    Apr 2011
    Location
    Norfolk UK and some of me is in Northern France
    Posts
    8,714
    OK In answering the anthropic minimum universe I got sucked into an unnecessary tangent. It seems to be begging the question to ask how this is the minimum necessary universe, when we cannot test any other universe until it pops into our experience. Thanks forthe corrections.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  31. #30
    Join Date
    Oct 2005
    Posts
    26,778
    You are raising an interesting question, which is, how can anthropic thinking help us understand the qualities of our universe? We know the physical constants have to be within a fairly small window to get intelligent life as we know it (of course other kinds of intelligent life is hard to even speculate about), but what about arbitrary initial conditions like the size of the universe and its expansion rate, and the amount of dark matter and dark energy? All of those have been talked about, and need to be pretty close to what we see, except for the size of the universe-- that really hasn't been talked a lot about, and maybe it could have been a lot smaller, or maybe it needs to be (and is) a lot larger than our observable corner. Given normal kinds of terms you see in the "Drake equation", it is often thought (though it's mostly a guess) that a single galaxy might be enough to get intelligent life. If so, then the observable universe is vastly larger than it needed to be, and that calls for some explanation.

    If one again thinks anthropically, you would have a kind of competition, where larger universes are less likely (i.e., appear less often in a distribution of universes), but have more opportunity for intelligent life. The opportunity is linear in universe size, so to get a peak in the product of size times likelihood, you need a likelihood that transitions from not much less likely with size to becoming less likely with size. An exponential like e-size/size_o seems like a likely choice, where size_o is an average size. But then an anthropic argument would hold that our universe size must be something like size_o in order for most intelligent life to find itself in a universe like ours (which is the core result of anthropic thinking). In short, the anthropic argument tells us nothing beyond that there should be a size distribution that puts our universe at a fairly average size, which doesn't tell us anything because anthropic thinking is never of value in regard to some universal parameter in any situation where it leads you to expect your universe to have a generic value of that parameter. (It's not valuable there because you would always think that, even if you don't use anthropic thinking.)

    In conclusion, I argue that anthropic thinking is not useful for helping us understand why our universe is so large, because it only works if we conclude that our universe is a typical size in whatever distribution sets that, and we would expect that anyway because people who think there is only one universe also think it is a standard size for a universe. The only time anthropic thinking about size would be useful is if our terms in the Drake equation make us expect that any intelligent life is quite unlikely anywhere in a universe the size of ours, because we would then need to think in terms of lots of universes to even understand why intelligent life happened once. Most people really don't think that intelligence should be that rare, so most would conclude that anthropic thinking is not useful when applied to the size of our universe-- something else must explain why it is so extremely much larger than it needed to be. So I think your question is quite interesting, given that it exposes we have a problem understanding the size of our universe even if we try to think anthropically: the "tuning" problem cannot be resolved that way. (I don't say "fine tuning," because that's just the issue-- the size of our universe is not fine tuned, so the question becomes, why not?)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •